Flow control around airfoils and wing sections is accomplished by two types of devices, those devices controlling the main flow and those controlling the boundary layer flow. The main flow devices include the airfoil shapes, flaps and flight control surfaces, such as ailerons, while the boundary layer control devices typically include wing fences, blowing or suction devices, vortex generators and the like. Although the main flow control devices produce the large scale effects which provide the necessary flight characteristics, often it is necessary to incorporate boundary layer control devices in order to allow the main flow devices to operate. For example, one can observe the presence of a large number of vortex generators mounted upstream of the trailing-edge flaps and ailerons on the wings of commercial aircraft. These devices are necessary to achieve high-lift characteristics during take-off and landing where the large flap deflections tend to cause extensive flow separation on the main wing. The absence of these boundary layer control devices results in much reduced lift because the trailing-edge flaps operate in an extensive separated flow region. While the vortex generators provide improvement in lift during the landing and take-off phases of flight, such devices are sources of drag throughout the remaining flight regime. This drag is caused by both form drag from the blockage caused by the device itself and by viscous drag, due to turbulent flow skin friction downstream of the device. Efforts to overcome these shortcomings include flap-mounted micro-vortex generators which extend and retract as the flaps are actuated. Shortcomings remain however, as the extension and retraction adds complexity and further provides no boundary control of the main wing when flaps are not extended. This interlinking of flaps and boundary layer control device means, for example, that an emergency flap-failure landing must be made at even higher approach and touch down speeds because both flaps and boundary layer control devices have been lost.
Additionally, many required applications of boundary layer control devices do not lend themselves to retractable vortex generators. For example, typical fixes for jet engine inlet flow problems have relied on inside-the-inlet vortex generators to mix the flow and prevent choked and stalled flow. Depending on the problems, such an installation may be extensive. An example is the General Dynamics F-111 swing wing fighter which had severe problems with engine stall associated with the poor flow through the inlet duct in the root section of the swing wing. The initial fixes included the installation of hundreds of vortex generator blades two to three inches long, around and along the inlet walls. This type of fix, although correcting the main flow problem, created a large boundary within the inlet duct greatly reducing its effective diameter. A boundary layer control device is needed which can operate passively, which will not include form drag or skin friction when boundary layer control is not needed, which will provide minimal blockage of the flow, and which operates independent of any control surfaces (flap, aileron, nozzle control, etc.)